1. Field of the Invention
The present invention relates to an antenna apparatus for use in a wireless communication apparatus which is controlled so as to achieve high-speed communication with increased communication capacity as well as to maintain high communication quality in mobile communication using a mobile phone or the like. More particularly, the present invention relates to a MIMO antenna apparatus and a wireless communication apparatus provided with the MIMO antenna apparatus.
2. Description of the Related Art
For an antenna apparatus adopting MIMO (Multi-Input Multi-Output) technique for simultaneously transmitting and/or receiving wireless signals in a plurality of channels using a plurality of antennas, there is a MIMO antenna apparatus disclosed in, for example, Japanese Patent Laid-Open Publication No. 2004-312381.
The MIMO antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2004-312381 includes four groups of antenna elements, each group equally spaced from the adjacent one, and a main body unit. Each group of antenna elements includes four antenna elements with different polarization directions. The main body unit includes: a switch unit connected to the respective antenna elements; a signal receiving unit for receiving received signals through the switch unit; an antenna controlling unit for generating a control signal for the switch unit; an antenna selecting unit for generating combinations of antenna elements and providing selected-element information to the antenna controlling unit; and an antenna determining unit for determining a specific combination of antenna elements based on received signals received by the antenna elements generated by the antenna selecting unit and providing determined-element information to the antenna controlling unit. This conventional MIMO antenna apparatus aims to, by means of such configuration, reduce a correlation between antenna elements to ensure sufficient transmission capacity, by determining combinations of antenna elements such that one antenna element is selected from each group of antenna elements.
Namely, in the MIMO antenna apparatus, if a plurality of antenna elements operate at the same time and each antenna element achieves the highest possible receiving power, it leads to an increased total transmission rate of a plurality of signal sequences after MIMO demodulation. The MIMO antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2004-312381 achieves the increased total transmission rate by providing it with a larger number of antenna elements than the number of MIMO simultaneous communication channels, selecting therefrom antenna elements having a higher receiving signal strength, and performing MIMO demodulation using the selected antenna elements. Such selection of antenna elements is particularly effective for the case of mobile communication, in which there are temporal variations in the signal strengths of principal polarization and cross polarization, or changes in the angle of arrival, due to the movement of a mobile station (user) or temporal changes in the surrounding environment. In addition, it is possible to cope with changes in polarization directions by using antenna elements with different polarization characteristics, overcome temporal variations by performing a control for switching antenna elements.
As described above, the MIMO antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2004-312381 is provided with a plurality of groups of antenna elements, each group including a plurality of antenna elements, and can select a combination of antenna elements with the lowest correlations or a combination of antenna elements with the highest transmission capacity by using a switch unit, to reduce the correlations between the antenna elements, thereby improving transmission capacity.
Furthermore, with reference to the Japanese Patent Laid-Open Publication No. 2003-87051, an example of an adaptive antenna apparatus including parasitic elements and variable impedance load elements will be described.
An adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2003-87051 has a structure in which the apparatus includes one feeding antenna element (referred to as “radiating element” in the Japanese Patent Laid-Open Publication No. 2003-87051), and a plurality of parasitic elements (referred to as “parasitic elements” in the Japanese Patent Laid-Open Publication No. 2003-87051) disposed around the feeding antenna element. Furthermore, to each parasitic element is connected a variable reactance element as a variable impedance load element. Each parasitic element is electromagnetically coupled to the feeding antenna element. By controlling reactance values of the variable reactance elements by an adaptive control type controller, a radiation directivity of the adaptive antenna apparatus can be changed. By means of such configuration, the apparatus aims to receive only a desired wave by suppressing interference waves arriving at a wireless transmitter/receiver. Thus, according to the adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2003-87051, it can be expected to control the directivity to achieve high quality wireless communication, by means of the feeding antenna element, the plurality of parasitic elements, and the variable reactance elements.
Furthermore, according to the adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2003-87051, it is possible to configure an adaptive antenna apparatus with one wireless communication circuit (e.g., a wireless transmitter/receiver circuit). In a portable wireless communication apparatus that operates by a rechargeable battery, particularly, including a mobile phone etc., a configuration with low power consumption is required so that the longest possible talk-time can be achieved. A standard adaptive antenna apparatus requires wireless communication circuits whose number is equal to the number of antenna elements, and thus requires high power consumption. However, according to the configuration described in the Japanese Patent Laid-Open Publication No. 2003-87051, an adaptive antenna apparatus is implemented with one wireless communication circuit (described as “demodulator” in the Japanese Patent Laid-Open Publication No. 2003-87051) by employing the control of the parasitic elements. Accordingly, both low power consumption and a small-sized configuration can be achieved.
As described above, the adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2003-87051 is configured with the feeding antenna element, the plurality of parasitic elements, and the variable reactance elements, and the adaptive antenna apparatus controls the variable reactance elements by the adaptive control type controller to change the directivity of the adaptive antenna apparatus, thus suppressing interference waves and controlling the directivity such that a beam is formed in a direction of a desired wave. Accordingly, an adaptive antenna apparatus enabling high quality wireless transmission can be provided.
The MIMO antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2004-312381 has the following problem. This conventional MIMO antenna apparatus includes, as described above, a larger number of antenna elements than the number of MIMO simultaneous communication channels, selects therefrom antenna elements having a higher received signal strength, and performs MIMO demodulation using the selected antenna elements, in order to achieve the highest possible received power. However, it is extremely difficult to mount a plurality of groups of antenna elements such as those described in the Japanese Patent Laid-Open Publication No. 2004-312381 on a small-sized device with a size of one wavelength or less, such as a mobile phone.
On the other hand, the adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2003-87051 has the following problem. This conventional adaptive antenna apparatus achieves a small-sized configuration by employing one feeding antenna element, and thus can be mounted on a small-sized device with a size of one wavelength or less, such as a mobile phone. However, since there is only one feeding antenna element, it is impossible to apply the adaptive antenna apparatus to a MIMO antenna apparatus that controls the directivity of each of a plurality of antenna elements for each of a plurality of transmitter circuits (or a plurality of receiver circuits). Namely, even if two feeding antenna elements are provided to the adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2003-87051, these two feeding antenna elements are electromagnetically coupled to all of the parasitic elements, and thus, even by changing the reactance values of the variable reactance elements, it is impossible to independently change the directivities of the two feeding antenna elements. Accordingly, the adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 2003-87051 cannot be used in a MIMO antenna apparatus.